This invention relates to reciprocation pumps and control circuits for them.
In one class of reciprocating pump, a piston continuously reciprocates in a cylinder to directly force a liquid from the cylinder, alternately pulling liquid into the cylinder through an inlet port from a reservoir and pushing it from the cylinder through an outlet port to the destination of the liquid.
In some uses of this class of pump, the pumps are designed to reduce pulsation in the flow of fluid. One such use is liquid chromatography. It is desirable in liquid chromatography that liquid which is pumped through a chromatographic column flow at a constant flow rate through the column so that different molecular species in the effluent from the column are eluted at times that are reproducible from run to run. Pulses in which the liquid flows at unpredictable rates reduce this reproducibility.
In one type of prior art pump of this class, the pressure at the outlet port of the pump is measured by a pressure sensor. A feedback signal from the pressure sensor controls the speed of the pump motor to cause the pump motor to react to changes in pressure in the chromatographic column and thus maintain a more constant rate of flow of the fluid. One pump of this type is described in U.S. Pat. No. 3,985,467, issued Oct. 12, 1976 to Peter Lefferson.
This type of pump has a disadvantage when used in liquid chromatography in that it maintains pressure constant against varying pressure loads but may cause the rate of flow of fluid through the chromatographic column to vary, even in applications where it is desirable to maintain the rate of flow of liquid constant.
In another type of prior art pump of this class, the piston is driven at a constant rate while expelling liquid from the pump into the chromatographic column, but when returning on a fill stroke to draw fluid into the pump from the reservoir, the motor is driven at an increased and substantially constant speed to draw the fluid into the pump more rapidly.
During the forward stroke of piston in this type of prior art pump, the piston moves at a higher than normal rate until the pressure in the pump cylinder equals the pressure that existed near the end of the liquid expelling foward stroke of the piston and just before the piston began a refill stroke. After the pressure in the cylinder reaches the pressure during constant flow rate pumping before the start of the refill stroke, the outlet valve is opened and the piston continues foward at a constant rate. This type of pump is described in U.S. Pat. No. 4,131,393 issued Dec. 26, 1978, to Haaken T. Magnussen Jr. and U.S. Pat. No. 4,180,375 issued Dec. 25, 1979 to Haaken T. Magnussen Jr.
This type of pump has several disadvantages such as for example: (1) the opening of the valve at the pressure of the last part of the previous cycle results in an increased time during which no liquid leaves the outlet port over that time needed to fill the cylinder; (2) the constant speed of the motor during refill and pump up does not reduce the time before fluid leaves the pump as soon as it could; (3) the pump is relatively uncomplicated because the acceleration time of the motor is time-limited rather than distance limited; (4) the pump is able to accomodate a wider range of flow rates without cavitation; (5) the pump motor maintains a constantly changing velocity during the refill portion of a cycle and a portion of a delivery stroke of the piston; (6) the average flow rate is continuously monitored and adjusted by adjusting a current input signal representing the preset flow rate of fluid; and (7) the velocity of the motor is increased to accomodate high pressure and high flow rates without long periods necessary to replace liquid last during a refill portion of a pumping cycle.